Next Mars Rover's Landing Site Narrowed to 4 Choices

NASA's Mars Science Laboratory, dubbed Curiosity, is now moving from problem child to a ready for action robot. Launch is set for 2011.

Credit: NASA/JPL

The latest Mars robot may be dead, but NASA scientists have
plenty to keep them busy as they scout out four potential stomping grounds for an
ambitious new rover pegged to be the next red planet explorer.

NASA declared the Phoenix
Mars lander ? its youngest Mars probe ? officially dead in late May after
photos taken of it from orbit revealed substantial damage from its environment
in the Martian arctic. Those photos came from the same powerful orbiter that
has been searching for the ultimate drop zone for NASA's new Mars
Science Laboratory (MSL) which is currently set for a November 2011 launch.

The new roving robot lab, known as Curiosity, is expected to
determine whether Mars is or was ever habitable to microbial life. The rover's
combination of technical improvements should make any potential landing sites
more scientifically rich than anywhere Mars landers have gone before.

"We will either land in
Disneyland or in the parking lot next to Disneyland," said Ashwin Vasavada,
Curiosity deputy project scientist at NASA's Jet Propulsion Laboratory (JPL) in
Pasadena, Calif., echoing the project's mission manager.? [Best
and worst Mars landings.]

Stomping grounds on Mars

NASA is closing in on the end of a long process ? which
began three years ago ? to poll? ?the Mars science community for potential
landing sites, then weigh the pros and cons of each.

Now, out of some 60 possible sites considered at different
stages of the process, the list has been whittled down to four. They are regions
of Mars known as Mawrth Vallis, Gale crater, Holden crater and Eberswalde
crater.

"These are the best places
you could possibly imagine you would want to go, and for the first time, you
can actually land near them and get to them," said Matthew Golombek of
JPL, co-chair of the Curiosity rover landing site steering committee.

Mars rover bull's eye

The Curiosity rover is the first Mars mission ever built to
use a guided entry, meaning it will steer itself through the Martian atmosphere
like a guided missile instead of flying passively like a shuttlecock. Because
of this, the spacecraft can hit a much smaller landing target than ever before.

NASA's Viking missions required landing zones 190 miles (300
km) long to account for possible drift as the craft descended through the
atmosphere. Having to find a safe area of that extent ? free of craters, cliffs
and rocks ? drastically limited the potential landing sites. ?Later, the Mars
Pathfinder mission and Spirit and Opportunity rover landings worked with target
ellipses 60 miles (100 km) long.

The Curiosity rover, however, is designed to hit a target just
over 15 miles (25 km) long and 12 miles (20 km) wide.

"It opens up a lot more
possibilities of squeezing the ellipse within the terrain and closer to
features of interest," said Vasavada. After landing, Curiosity will also
be able to drive up to 20 kilometers to reach targets, or "go-to"
sites.

The new rover is also built to decelerate faster than
previous missions, meaning it can land on more elevated terrain, which opens up
even more of the red planet for exploration. And because Curiosity lacks solar
panels and is designed to withstand severe cold temperatures, mission planners
can target anywhere from thirty degrees North to thirty degrees South latitude.

Where to land on Mars?

These broader landing possibilities give researchers roughly
half the planet to choose from, or triple the area open to Spirit
and Opportunity rovers. To zero in on the most interesting landing sites,
Mars scientists made use of data collected from orbit.

In 2005, the OMEGA spectrometer on board ESA's Mars Express
orbiter picked up the presence of phyllosilicates ? clay minerals that formed
in the presence of water ? on the Martian surface. For the past few years, Mars
Reconnaissance Orbiter (MRO) has been mapping those phyllosilicates at high
resolution.

All four of the landing sites under consideration contain
layered phyllosilicates, which must have been deposited over a period of time.
Vasavada said the phyllosilicates are all found in terrain that dates back to
roughly the first billion years of Mars history, known as the Noachian era.
Curiosity will be the first rover to land on such ancient terrain.

Ideally, Mars scientists would like a landing site in which
they know going in how the phyllosilicates were formed, to be sure they can
construct a complete story of the features Curiosity will be examining.

The most comprehensive of possible landing sites for the
Curiosity rover are Holden crater and Eberswalde crater, adjacent to each other
in the Southern highlands of Mars.

Based on MRO observations, Holden crater was formed from an
impact that disrupted an existing river system. The river eventually broke
through the crater and flooded it, leaving behind phyllosilicate minerals.
Holden may have also contained lakes prior to the flooding, which would have
left their own traces.

Eberswalde predates Holden and
contains something very exciting to researchers ? the sedimentary remains of a
river delta on the western side of the crater.

"Here the geologic story is
really tight," Vasavada said. "It's basically a pile of sediment that
was deposited by a river."

Curiosity could be used to
figure out how long the river system was running, what kind of sediment was
deposited and where it came from. River deltas trap sediment along with
organisms and organic molecules, which are all the things mission scientists
want to study.

"The downside,"
according to Vasavada, "would be that the delta may have only been active
for very short time in Mars history."

Riskier Mars targets

Riskier from the perspective of geological understanding but
potentially more rewarding in terms of mineral remains are Mawrth Vallis and
Gale crater.

Mawrth Vallis is one of the oldest valleys on Mars, formed
approximately 3.7 billion years ago. It also contains the best-exposed
phyllosilicates on Mars.

In images from MRO's CRISM instrument, scientists have
identified multiple layers of phyllosilicates made of different materials. Some
are rich in iron and magnesium; others are rich in aluminum.

"It suggests a change in environment, a change in
chemistry" when Mars was warmer and wetter, said Golombek. "All of
that is incredibly interesting to go look at."

What Mawrth Vallis lacks is an understanding of how the
minerals got there, Vasavada explained. "That's something we could
discover by putting a rover there and driving around, [but] we would like to
know that before."

A similar situation holds for Gale, a 90-mile (150-km) wide
crater near the bottom of Elysium Planitia.

Some 3.5 to 3.8 billion years old, Gale contains a mound of
phyllosilicates in the middle that towers 3 miles (5 km) high ? higher even
than the crater's rim. At the top of the mound are younger sulfate minerals,
which would have formed in more acidic water than the phyllosilicates below
them.

If Curiosity landed at Gale, it might be able to survey a
billion years of Mars history, Vasavada said. The only problem, he added, is
that researchers can't be sure whether the minerals were deposited by water or
air.

"Everybody's wishing we had the perfect site where we
could sort of do it all," said Vasavada. "Really there are strong
supporters behind each of these four sites for different reasons."

More debate ahead

At the next landing site workshop in September, researchers
will continue debating the pros and cons of the four sites, including how long
it will take Curiosity to reach the phyllosilicates in each case and the
potential pitfalls along the way such as cliffs and rocks. "If it takes a
year to drive out of the ellipse, that's a factor you have to take into
account," said Golombek.

The final recommendation won't come until next May or June,
after MSL scientists construct simulations of the surface conditions following
a fifth workshop next March or April. After that, it will be up to NASA's
Associate Administrator for Space Science to make the final decision.

It's too soon to make an informed choice, said Golombek.
"The science is only half of the issue. I don't think we fully understand
all the potential hazards yet."